As a means for estimating conditions of a transmission line of broadcasting information in digital broadcasting reception based on the orthogonal frequency division multiplexing (called “OFDM” from now on) which uses multiple carriers and transmits information discretely, delay profile calculation is known which represents characteristic delayed wave received power against delay time. The delay profile can provide information about a currently receiving incoming path (transmission line) environment. Accordingly, in a multipath environment having delayed waves mixed due to reflections from mountains and the like or in a multipath environment caused in an SFN (single frequency network), the delay profile can be used as a means for determining a starting position of an FFT (Fast Fourier Transform) window that will prevent the delay path components from bringing about inter-symbol interference, that is, that will place all the path components within a guard interval range.
As conventional technology concerning the delay profile, the following are known.
A conventional example 1 of the technology aims at obtaining the propagation conditions of the information transmission line by using an existing radio wave without using a special radio wave or receiver. It has such a configuration that performs FFT processing on a baseband signal with an FFT (Fast Fourier Transform) operation unit to transform it to a frequency axis signal, and extracts with a pilot signal extractor only a scattered pilot (SP) signal (called simply “pilot signal” from now on) which is used for amplitude/phase equalization and is disposed on a frequency axis, from the frequency axis signal output from the FFT operation unit. Subsequently, using an amplitude/phase frequency characteristic detector, it interpolates the pilot signal for the amplitude/phase equalization, and generates and outputs frequency characteristic signals concerning the amplitude and phase, respectively. Besides, using an IFFT (Inverse Fast Fourier Transform) operation unit, it obtains a time axis signal of the output of the amplitude/phase frequency characteristic detector, and detects the delayed wave received power against the delay time. It displays the power thus obtained on a display unit in a predetermined format as a delay profile, and stores in a data storage if necessary (see Patent Document 1, for example).
A conventional example 2 of the technology aims to improve the quality of a received signal when receiving a digital broadcast based on the OFDM modulation by controlling an FFT window to an appropriate position quickly even when receiving a delayed wave including a preghost signal. It has such a configuration that implements a digital broadcasting receiving apparatus which includes: an FFT section for performing a Fourier transform by extracting an effective symbol period from transmission symbols of the OFDM signal; a guard correlator for obtaining correlation between the OFDM signal and its delay signal and outputting guard correlation results; a delay profile calculating section for performing inverse FFT of the pilot signal of the OFDM signal to obtain the delay profile; and a timing signal generator for mixing a signal corresponding to the guard correlation results and a signal corresponding to the delay profile, and for generating a timing signal for controlling the calculation range according to the resultant signal of the mixing (see Patent Document 2, for example).
Patent Document 1: Japanese patent application laid-open No. 2000-115087.
Patent Document 2: Japanese patent application laid-open No. 2004-96187.
The conventional delay profile generating technology in the OFDM digital broadcasting reception, and the application technology of the delay profile are configured as described above. They offer the following advantages: the former can obtain the propagation conditions of the information transmission line without using a special radio wave or receiver; and the latter can improve the quality of the received signal. Both the technologies, however, cannot cope with an environment in which the digital broadcasting receiving apparatus is mounted on a mobile vehicle traveling at a high speed.
When the digital broadcasting receiving apparatus is mounted on the mobile vehicle traveling at a high speed, unexpected “ghosts” appear which differ from the normal “delayed waves” (including the foregoing “preghosts” as well) appearing under the multipath environment. Such ghosts, which do not appear at standstill, have a property of increasing their intensity up to a level comparable to the primary wave (true signal) as the speed of the mobile vehicle increases.
Thus, even if it produces the delay profile under the high-speed transfer environment, the conventional example 1 with the foregoing configuration cannot detect the presence of the ghosts, which presents a problem of degrading the estimation accuracy of the conditions of the transmission line.
In addition, since the ghosts differ from the normal delayed waves as described above, when the conventional example 2 with the foregoing configuration carries out the FFT window control, ghosts will change the FFT window control state. As a result, it provides a problem of bringing about a malfunction, thereby causing radio interference.
The present invention is implemented to solve the foregoing problems. Therefore it is an object of the present invention to provide a digital broadcasting receiving apparatus with a channel estimation function capable of improving, when the digital broadcasting receiving apparatus for receiving an OFDM digital broadcast is mounted on a mobile vehicle traveling at a high speed, the receiving performance of a broadcast signal by generating the delay profile in the same manner as the conventional technology, by improving the estimation accuracy of the conditions of the transmission line by enabling the presence/absence decision of the ghosts from the delay profile generated, and by carrying out the FFT window control with taking account of the ghost decision results.